block: don't lose data from last incomplete sector

To read the last sector that is not aligned to sector boundary, current
code for growable backends, since commit 893a8f62 "block: Produce zeros
when protocols reading beyond end of file", drops the data and directly
returns zeroes. That is incorrect.

Signed-off-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>